This present invention relates to pumping assemblies, and finds particular application in pumping cryogenic materials, for example, where the pump assembly is immersed in fluid stored in a reservoir or container, such as a transport ship, and is required to pump the fluid from the bottom of the reservoir.
Pumps that embody inducers for liquid natural gas (LNG) applications such as LNG carrier loading pumps and primary send-out pumps are often required to operate at very low values of net positive suction head required (NPSHR) to facilitate the complete stripping of the storage tanks while maintaining full flow even while operating in full cavitation mode. Additionally, while operating at low tank levels, the pumps can ingest vapors caused by poor suction conditions and vortices. This results in two-phase flow regime.
Under such conditions, inducers in LNG pumps need to be capable of developing sufficient head (pressure) to compress these vapors sufficiently for reabsorption into the liquid in a hydrodynamically stable way. Otherwise it is a well known fact that the pump discharge pressure fluctuates when a column of vapor enters the pump inlet that is not fully reabsorbed. The presence of such fluctuations can cause vibration that can shorten pump life.
U.S. Pat. No. Re 31,445, the details of which are incorporated herein by reference, is directed to a submersible pump assembly of the type for which the improved inducer or high performance inducer was developed. The '445 patent discloses a cryogenic storage system in which a reservoir, storage tank, tank car, tanker ship, etc., includes a casing suspended from an upper closure member or roof. Pipe sections extend from the roof and house a pump and motor unit that is positioned on a floor of the reservoir or storage container. Power is provided through electrical cables and the entire pump and motor assembly is suspended via cable or rigid tubes or pipes.
A foot plate is provided on the lowermost end of the pump and motor assembly. Disposed inwardly from the bottom end is a flow inducer vaned impeller. As described in the '445 patent, a typical inducer impeller includes plural, circumferentially spaced vanes that extend radially outward from a central hub. This structure is generally referred to as a fan-type inducer. Still other manufacturers use a different impeller or inducer configuration such as a mixed flow inducer rather than the four blade fan-type inducer shown in the '445 patent.
Although known fan-type inducer and mixed flow inducer pumps have been used with some success in pump assemblies of this type, they encounter the above-described problem when used to pump a two-phase medium or fluid (i.e., liquid and vapor). As more air than liquid is drawn into the pump assembly because of the design, a substantial amount of the fluid is left in the reservoir. If LNG is shipped in a transport ship, for example, it is offloaded or pumped to a storage reservoir on shore. The inducer is an important element that needs to operate where very low inlet pressure is available. In LNG loading and primary send-out pumps, these conditions exist because the liquid in the tank is at or near saturation pressure (also referred to as true vapor pressure) when the level in the storage tank provides little submergence. In LNG secondary send-out pumps, these conditions can exist because the recondenser is at true vapor pressure when the pipe losses from the boil-off gas recondenser and the pump suction approach the elevation difference between the free liquid surface in the recondenser and the pump inlet (inducer eye).
When these conditions occur, the pressure in the inducer eye becomes equal to true vapor pressure, and any further pressure reduction will result in cavitation, producing bubbles or clouds of bubbles in the fluid. This occurs at the leading edge of the inducer blade when the relative velocity of the fluid with respect to the blade has any incidence angle other than zero. Under other conditions, vapor clouds can be ingested by the pump when suction vortice funnels open between the pump suction and the fluid free surface allowing a stream of vapor to flow into the pump suction. The ratio of vapor to liquid by volume is referred to as V/L or void fraction. The liquid/vapor mixture is two-phase flow. In extreme cases, clouds of bubbles or voids will block the flow and reduce pump output and efficiency.
Known inducer designs leave approximately four feet of LNG in the base of the reservoir of the transport ship. In other words, the reservoir of the ship is not sufficiently emptied and the transport ship is forced to carry residual LNG from the pumping station to a remote location where the transport ship is subsequently refilled. It is estimated that costs associated with this undesired retention and needless shipping of residual LNG that is not pumped from the transport container can cost approximately one hundred thousand dollars ($100,000) per year per foot of residual LNG.
In light of the foregoing, it becomes evident that there is an appreciable need for an improved high performance inducer assembly that would provide a solution to one or more of the deficiencies from which the prior art has suffered. It is still more clear that an improved high performance inducer assembly providing a solution to each of the needs inadequately addressed by the prior art while providing a number of heretofore unrealized advantages thereover would represent a marked advance in the art. Accordingly, a need exists for an improved high performance inducer assembly and particularly an improved high performance inducer to significantly reduce the amount of residual LNG remaining in the ship reservoir after pump off. Likewise, a need exists for more efficient handling or pumping of a two-phase fluid.